Beta-Phase Poly(vinylidene fluoride) Shish-Kebabs Produced under Shear and Pressure with High Piezoelectric Response

Poly(vinylidene fluoride) (PVDF) was crystallized under a combination of shear rate (0–31 s–1), pressure (50–600 MPa), and temperature (150–200 °C), producing flexible pressure sensor films with a high piezoelectric stress coefficient g33 of up to 1.2 V·m·N–1. This matches the highest value reported in the literature, but is achieved more quickly, at lower temperatures, and at significantly lower pressures. The key was to overcome the entropically favored gauche-rich centrosymmetric alpha form and obtain almost exclusively the trans-rich piezoelectric β and γ forms. This was achieved by steering the system through the highly supercooled hexagonal mesophase, normally achievable only at high pressures, but here, aided by shear, was transiently reached at only 1 kbar (100 MPa). A further advantage of the highly oriented shish-kebab structure that results from this process is that the favorable molecular dipole orientation is intrinsically higher compared with that in the normal spherulitic structure. A qualitative, thermodynamics-based mechanism is proposed, explaining the synergism of flow and pressure. The films were obviously poled, although no special poling procedure was applied. The “self-poling” was attributed to the temperature gradient in the pressure-shear cell. Global symmetry breaking is assumed to have occurred at the transition from the hexagonal mesophase to the crystalline beta form.